I can only state one thing, based on observing of all the people's complaints about the changes in Avast Firewall.
Windows Firewall (default one) has a more in-depth control over the creation and editing of the existing rules.
There's a big flexibility in the Windows Firewall. And there's none in Avast.
For common users it might be a good thing.
But for advanced users it's horrible!
At least, there should be a switch to advanced mode, in Avast's settings that allow to configure Firewall the same in-depth as Windows Firewall allows, and edit the rules. That way it will appeal to anyone, and not just to common users, who might not even know the difference between Firewall and Antivirus all that much.
And I really doubt that protection is that much better in Avast Firewall to justify its usage over the default firewall.
The simplest reason for such doubts, is how huge the userbase is for the default Windows Firewall (over the course of all Windows versions) than for Avast Firewall.
So they could get more chances to get more bug reports, catch problems, fix things.
Windows Firewall, most likely, is the most reliable out of most alternative products.